1. Field of the Invention
The present invention relates to a method for preparing a multilayer of nanocrystals, and an organic-inorganic hybrid electroluminescence device comprising a multilayer of nanocrystals prepared by the method. More particularly, the present invention relates to a method for preparing a multilayer of nanocrystals by (i) coating nanocrystals surface-coordinated by a photosensitive compound, or a mixed solution of a photosensitive compound and nanocrystals surface-coordinated by a material miscible with the photosensitive compound, on a substrate, drying the coated substrate, and exposing the dried substrate to UV light to form a first monolayer of nanocrystals, and (ii) repeating the procedure of (i) to form one or more monolayers of nanocrystals on the first monolayer of nanocrystals; and an organic-inorganic hybrid electroluminescence device using the multilayer of nanocrystals prepared by the method, as a luminescent layer. The use of the multilayer of nanocrystals as a luminescent layer can enhance luminescent efficiency and luminescence intensity of the electroluminescence device, and can control electrical properties of the electroluminescence device.
2. Description of the Related Art
A nanocrystal is defined to be a material having a crystal size at the nanometer-scale level, and consists of a few hundred to a few thousand atoms. Since the nanocrystal has a large surface area per unit volume, most of the atoms constituting the nanocrystal are present at the surface of the nanocrystal. Based on this structure, the nanocrystal exhibits quantum confinement effects, and shows electrical, magnetic, optical, chemical and mechanical properties different from those inherent to the constituent atoms of the nanocrystal. That is, the control over the physical size of the nanocrystal enables the control of various properties.
Vapor deposition processes, such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), have been conventionally used to prepare nanocrystals. On the other hand, a wet chemistry technique wherein a precursor material is added to an organic solvent to grow nanocrystals to a desired size has made remarkable progress in the past decade. According to the wet chemistry technique, as the crystals are grown, the organic solvent is naturally coordinated to the surface of the quantum dot crystals and acts as a dispersant. Accordingly, the organic solvent allows the crystals to grow to the nanometer-scale level. The wet chemistry technique has an advantage in that nanocrystals can be uniformly prepared in size and shape in a relatively simple manner at low cost, compared to conventional vapor deposition processes, e.g., MOCVD and MBE. However, since nanocrystals prepared by the wet chemistry technique are commonly dispersed in an organic solvent, techniques for forming a thin film of the nanocrystals are required in order to apply the nanocrystals to electronic devices.
Conventionally, a self-assembly process has been mainly used to form a thin film of nanocrystals prepared by the wet chemistry technique. For example, U.S. Pat. No. 5,751,018 discloses a method for attaching semiconductor nanocrystals to solid inorganic surfaces, using self-assembled bifunctional organic monolayers as bridge compounds. Further, Korean Patent Application No. 2002-85262 discloses a method for preparing a multilayer of nanocrystals by bonding a dithiol group to the surface of nanocrystals to form disulfide bonds between the nanocrystals. Further, a method is disclosed for preparing a multilayer of nanocrystals by charge-charge interaction. According to this method, nanocrystals surface-substituted with a charged compound are bound to an oppositely charged substrate to form a thin film of the nanocrystals, an organic compound oppositely charged to the nanocrystals is bound on the thin film, and then the above procedure is repeated.
However, these methods for forming a thin film of nanocrystals by self-assembly have a problem that they involve an additional step of treating the surface of nanocrystals and a substrate with a compound containing a specific functional group, rendering the overall procedure more complex.
Meanwhile, electroluminescence devices using nanocrystals as a luminescent layer are described in U.S. Pat. Nos. 5,537,000, 6,608,439 and 6,049,090, and PCT publication WO 03/084292.
U.S. Pat. No. 5,537,000 describes an electroluminescence device without an organic electron transport layer in which a multilayer of nanocrystals acts as both a luminescent layer and an electron transport layer, and the wavelengths of emitted light are dependent on a voltage applied to the device. However, the patent publication simply describes that the multilayer of nanocrystals can be prepared by the use of a self-assembly process mentioned in U.S. Pat. No. 5,751,018, but fails to specifically describe a process for forming a monolayer of nanocrystals or preparing a multilayer of nanocrystals using the monolayer.
PCT publication WO 03/084292 describes an organic-inorganic luminescence device which comprises a matrix containing a large number of nanocrystals and disposed between two electrodes. This patent publication also suggests a method for enhancing electroluminescent performance of the device by spin-coating a mixture of nanocrystals and a low-molecular weight hole transporting material (e.g., N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)4,4′-diamine (TPD)) on an electrode, and introducing an organic electron transport layer and electron/hole blocking layers into the device, thereby allowing the organic layer to transport electrons and holes and allowing the nanocrystals to emit light.
Further, U.S. Pat. No. 6,608,439 discloses an integrated organic light emitting diode color display device in which nanocrystals used as a color-conversion layer absorb monochrome and short-wavelength light emitted from an organic layer, and then emit photoluminescence (PL) at a different wavelength. However, the luminescence device is not driven by electroluminescence.
Further, U.S. Pat. No. 6,049,090 describes a device using a mixed layer of nanocrystals-matrix as a luminescent layer disposed between two electrodes wherein the band gap energy and the conduction band energy level of the matrix are more than those of the nanocrystals. The patent publication also explains that since electrons and holes are trapped by the matrix, the luminescent efficiency of the device can be enhanced.
As described above, there are few reports on the preparation of a multilayer of nanocrystals used as a luminescent layer in order to enhance the luminescent efficiency of electroluminescence devices, except for the self-assembly process involving complicated steps.